1. Field of the Invention
The present invention relates to a laser range finder, and particularly relates to an optical system for a laser range finder.
2. Description of Prior Art
A laser range finder is one of the main devices for distance measurement. A common type of laser range finder usually applies a laser emitter as a light source for transmitting a modulated laser light beam to a target object to be measured. The target object reflects and returns the laser light beam to a laser receiver, which is commonly an avalanche photo diode (APD) to convert the optical signal into an electric signal. The distance to the target object is determined by multiplying the light velocity by the time interval between the pulse emission time and the reflected beam reception time.
However, as the light velocity is an extremely large value, the processing of electric signals must be very careful and accurate to obtain an accurate time value, so that the accuracy of the distance to the target object may be ensured. Additionally, the light has the diffusibility characteristic, and thus the target object can only reflect a portion of the transmitted light for reception by the laser receiver. The reflected light is reduced to a little amount especially when no auxiliary cooperative target is provided. Furthermore, the ambient rays and dust particles etc. may interfere with or influence the reflected light, causing reduction of the signal to noise ratio, thereby affecting the measuring precision of the range finder. One solution to this problem is to increase the laser transmitting power, which results in increased cost and hurt to the operator's eye.
Distance measurement by laser encounters many technical difficulties in short distance and high precision applications. These technical difficulties include the reception of the reflected light from a target object at a short distance, the acceptable light amount limit of the APD, the requirement of equal optical path length for high precision distance measurement, and so on.
One conventional solution in this aspect is to adopt a light guide. The measuring light beam reflected from the target object is received by a reception objective lens, and is converged on and coupled to the light guide. The reflected light beam is first transmitted along the light guide for a predetermined distance, then coupled to a small lens, and finally converged on the APD. The advantage of light guide transmission consists in the fact that the position of the APD may be adjusted and it is also suitable for laser distance measurement in the case of short distances to the target object. In respect of reception of the reflected light beam from a near target object, U.S. Pat. Nos. 5,815,251 and 5,949,531 propose various solutions as respectively shown in FIGS. 1-4. FIG. 1 illustrates a first solution which uses a motor driven eccentric 12 to displace a leafspring 13, whereby the position of the light guide mount 14 with a light guide entry surface 16 on the right side thereof may be transversely adjusted correspondingly. The light energy is than transmitted to an optoelectronic converter 15 to achieve the reception of the reflected light beam from the near target object. FIG. 2 shows a second solution which uses a planar mirror 21 for reflecting the incident light reflected from the near target object at large incident angles on the light guide entry surface 16 and then to the receiver. However, this proposal may cause bending and scattering of light to some extent. A third solution is shown in FIG. 3, which uses a prism 31 to refract the reflected light at large incident angles in the case of short distances to the target object. One problem with this proposal is that the prism 31 may also refract the reflected light from some distant target objects, which results in insufficient light energy for the light guide. Consequently, the prism 31 must be removed in the case of relatively large distances to the target object. FIG. 4 illustrates a fourth solution that uses a diffractive element 41 applicable to the reflected light from different directions. One disadvantage for this solution is that the configuration of the diffractive element 41 is relatively complex, and thus is not cost-effective.
Although the above-mentioned conventional solutions are applicable to some actual applications, no consideration can be found in the above references concerning the efficiency ratio of the reflected light to the emitted light and the requirement of equal optical path length as well, whereby the signal accuracy thus cannot be increased. Secondly, some solutions require motor driving, or require insertion/removal of a specific element into/from the optical system when measuring different distances to near or distant target objects. These additional configurations or elements bring instability to the system, thereby decreasing the reliability of the system and also increasing the manufacturing cost of the system.